Imagine a bike and a rider that weight 100 Kg. The weight distribution is 60% rear 40% front for example.

When the rider brakes, the weight distribution changes because of inertia (40-60 30-70... it depends on how strong is the deceleration). Most of the weight goes to the front, so the fork dive and the rear end rise.

A2= 100% means that the braking force is trying to compress the rear suspension with the same force as the inertia is trying to rise the rear end so they cancell each other and the rear end stays the same (The fork still Dives).

If A2>100% the Squat Force is bigger than the weight shift so you get some compression on the rear end. If A2<100% you still have some squat force but it's smaller than the weight shift so the rear end is going to rise a little bit.

With less A2 you have more grip at the tire, and the bike tracks the terrain better.